A number of integrated circuits are typically fabricated at the same time on a common slice of silicon or wafer. This wafer is usually substantially circular with a diameter of around 3 to 6 inches (7.5 to 15 cm). Once the fabrication process is complete, the wafer is then sliced-up into the individual integrated circuit (IC) chips or dice (singular: die) which are later packaged into modules or incorporated into larger systems.
This process of dividing a wafer into its individual dice involves scribing the wafer with grooves, then breaking the wafer along those grooves like pieces of a chocolate bar. Alternatively, the dice are cut from the wafer using a saw such as a laser. Sometimes, the entire dividing process is referred to as "scribing". When the dice are still connected as an integral wafer or separated but closely packed together as if they still were a wafer, the IC's are said to be in the wafer scale of development.
The steps taken to create a finished, packaged integrated circuit are exhaustive. Consequently, the earlier one can discover a faulty device the better. This device can then be thrown out of any further processing, thereby increasing throughput, saving time and money.
Most IC's that are going to fail before a reasonable lifetime, do so at an early stage. Therefore, these units may be screened out by running all the IC's for a time and then testing prior to shipment. This weeding out process can be hastened by elevating the temperature and applying voltages either statically or dynamically to the IC's. This process is called burn-in. In the past, this required placing the packaged IC module into a special oven or autoclave capable of applying the proper burn-in voltages or signals.
Since some IC's are designed or required to operate in a cooled environment, adequate testing may require special cooling apparatus. In the past, this involved placing the module to be tested inside a cooling chamber or employing highly specialized test equipment.
Often times, more than one IC is packaged in a single module called a Multi-Chip Module (MCM). In this case, burn-in and testing can be more difficult due to the greater complexity of the total circuitry. Also, the faulty module may need to be scrapped or repaired, both of which are time consuming. It is advantageous therefore, to burn-in and test IC's prior to packaging. Once this is feasible, as it now is using the invention, a chip fabricator can supply the MCM market with burned-in and tested dice.
Currently, there is no wafer scale burn-in capability in the industry. Since a single wafer contains hundreds or thousands of IC's held together in precise alignment, it would be desirable to take advantage of this situation for burn-in. With a single alignment of the wafer, it would be possible to burn-in and test all the dice in parallel. As with any wafer scale system, the close proximity of the dice to each other allows for greater speed and less power consumption.
Currently, wafer scale testing of IC's involves using a test probe. In a time consuming procedure, the probe must be precisely aligned with each die to be tested, one at a time. Since this testing is done prior to burn-in, the time spent testing a future failing die is wasted time. Currently, the efficiency of a test probe is limited by the number of contact pads it can engage at any one time.
The next logical step in miniaturization for the semiconductor electronics industry involves wafer scale integration. Basically, this means closely packing the dice used in a system so that they have the same density they enjoyed when they were connected as a wafer. The ability to burn-in and test an entire wafer would be invaluable in implementing this scheme effectively. This would also allow testing the circuitry while the dice are inter-connected and operating in concert.
FIG. 1 shows the steps involved in processing a fabricated wafer through shipment using the current state of the art process flow, with burn-in occurring after scribing and packaging. This of course applies to the typical manufacturer who ships packaged modules. Shipment can occur at any stage depending on what product or service the manufacturer or company provides.
Typically, the fabricated wafer is first tested 1 using a probe which tests selected portions of the wafer. If any portion is determined to have a repairable fault, repair is implemented by blowing fuses which activate redundant circuitry. Fuses are blown either by laser or electrical means, after which the wafer is retested.
The wafer is then scribed 2, thereby dividing the wafer into individual dice. The dice are attached and wire bonded to a leadframe, wherein each die is packaged 3 by encapsulation into a module. The leads of each module are trimmed, formed and soldered.
The finished modules are then sent to intelligent burn-in 4 and tested to eliminate infant mortalities. Intelligent burn-in is different from normal static or dynamic burn-in because it simulates an operating environment and tests the module during burn-in. The devices are then speed graded to discover their characteristics and tolerances, then categorized as to quality. For example, it may be worthy of military rather than commercial applications.
The modules are then sorted and marked 5, according to their quality and function, then finally tested 6 prior to shipment.